rosin (or resin) is used on bow hair to make the tiny "barbs" of horse hair stand up, therby making them act as little picks or plectrum as they are drawn across the string. By making sure the hair that is being used is real (I'm told bleached Chinese horse tail hairs are the best) and by scratching the surface of a brick of rosin (Pirastro is one of the best, I am told) and carefully pulling the bow across the brick (from tip to frog and back again), the bi-directional layout of the hairs ( if applied properly in the first place) will work with a minimum of rosin-usually 4 or 5 strokes. Well that is about all I know about that, please keep in mind that I am a sax player and what I know about the fiddle would only fill a capful! Best Wishes-Jack.